G. Mirshekari

Innovative thermal energy harvesting for zero power electronics

Thermal gradients, commonly present in our environment (fluid lines, warm fronts, electronics) are sources of energy rarely used today. This paper aims to present innovative approaches of thin and/or flexible thermal energy harvesters for smart and autonomous sensor network applications. The harvester system will be based on the collaborative work of interrelated energy nodes/units, which will be either piezo-thermofluidic converters (use of rapid thermal cycles of a working fluid) or piezo-thermomechanic converters (use of the mechanical energy developed by rapid snapping of micro-switches). The two kinds of energy nodes convert a heat flux into storable electrical energy through a piezoelectric transducer. Miniaturization of the energy nodes will lead to increased thermal transfer rates and consequently increased harvested power. To effectively use thermal energy sources in varying environments, the nodes will be adaptive versus different thermal gradients (in a predefined temperature range) and will possibly influence each other. The concept is unique in the sense that it is based on a matrix structure of micro or mini energy nodes which will work together in a collective approach to optimize the harvested energy, and which do not require the use of radiators as classical Seebeck approach, thanks to the controlled thermal resistance. This opens the door to new properties and features of the object, with better performances. It could therefore be declined on flexible substrates, allowing conformability around the sources of potential heat for low power applications.

Microscale Shock Tube

This paper reports on the design, microfabrication, characterization, and testing of the first instrumented micrometer-scale shock tube. This device was fabricated by a series of etching, deposition, and patterning processes of the different structural layers on a silicon substrate to first create an array of direct-sensing piezoelectric pressure sensors followed by the bonding of another substrate to create a microchannel. The resulting assembly is a rectangular channel with a hydraulic diameter of 34 μm and a length of 2000 μm, instrumented with five wall pressure sensors along its length. This device is used to characterize, for the first time, the propagation of shock waves at microscales, where transport effects such as wall friction and heat transfer are important. The results show shock-wave attenuation along the length of the microchannel in accordance with simple analytical models for these flows.

Numerical Study of the Shock Wave Propagation in a Micron‐Scale Contracting Channel

Entry of a shock wave into a microchannel and its propagation in the channel are studied numerically by the continuum and kinetic approaches. It is shown that the shock wave is amplified immediately after it enters the microchannel. After that, the shock wave in an inviscid computation propagates over the microchannel with a constant velocity. In a viscous computation, the shock wave velocity decreases and the wave attenuates. Qualitative agreement between experimental data and viscous computations is demonstrated.

THROUGH SILICON VIAS INTEGRABLE WITH THIN-FILM PIEZOELECTRIC STRUCTURES

This paper reports on the design and microfabrication of novel through silicon vias (TSV) that are compatible with high-temperature processing of piezoelectric structures. The present approach uses metal deposition in cavities etched in the SOI handle layer of the wafer and electrically isolated islands in the device layer. This design avoids the shortcomings of previous TSV designs, which either introduce large topologies on the wafer surface, include metals that cannot sustain high-temperature processing or use poor electrical insulators. TSVs microfabricated using this new approach exhibit good performance, specifically small resistance between the front and backside metal pads, isolation from the ground plane and small capacitance between the vias and the ground. These TSVs are eminently suitable for devices requiring high-temperature processing, such as thin-film piezoelectric sensors and actuators.

Piezoelectric Pressure Microsensor Arrays for the Simultaneous Measurement of Shock Wave Amplitude, Velocity and Direction at a Point

Because of their fast response, piezoelectric pressure sensors are commonly used in applications involving acoustic or shock wave propagation to provide an accurate time-history of pressure at a certain point in the flow. However, it is often necessary to probe the flow at different locations, so that, for example, the direction and the speed of the waves can be inferred.

Numerical Simulation of Shock Wave Entry and Propagation in a Microchannel

The effects of viscosity and heat conduction, heat losses due to the wall heat transfer, as well as nonequilibrium phenomena can play an important role in microflows. Recent numerical investigations [1] of shock wave propagation in a microchannel with allowance for viscosity and rarefaction effects revealed significant differences from the inviscid theory, which ensures a correct description of the majority of specific features of macroflows. In that work, the shock wave was generated by breakdown of a diaphragm separating high-pressure and low-pressure domains.

Flow Phenomena in Microscale Shock Tubes

Recent experiments on small shock tubes, at normal or reduced pressure, have revealed interesting phenomena. For example, Brouillette [1], using pressure instrumentation, has examined the operation of a 5.3 mm shock tube at initial pressures down to 1 mBar for diaphragm pressure ratios up to 105 and found a decrease in shock strength with decreasing scaling parameter ReD/L for a given diaphragm pressure ratio. Garen et al. [2], with an atmospheric 1 mm shock tube that used a quick opening valve instead of diaphragm, reached the same conclusions using laser differential interferometry observations.

Compressible Microchannel Flow

We present experiments on the isothermal gas flow at relatively high Mach numbers in microfabricated channels of small aspect ratios. The microchannels were fabricated by deep etching on silicon wafers, bonded to a Pyrex wafer to cover and seal them; the microchannels were 10 microns deep with a variety of widths. The accurate determination of the small flow rates was performed by measuring the displacement of a bead of mercury in a precision bore glass tube in a controlled environment. The experiment setup has been specially designed to account for inlet and outlet loss. The inferred friction coefficient at different values of Knudsen, Reynolds and Mach numbers shows that the flow inside the microchannel follows the classical laminar behavior over the range of experiments.Copyright

Progress towards a microfabricated shock tube

This paper presents the progress in the design, fabrication and testing of a microscale shock tube. A step-by-step procedure has been followed to develop the different components of the microscale shock tube separately and then combine them together to realize the final device. The paper reports on the progress in microfabrication of the microchannel and associated pressure sensor system.